The crystalline lens of the human eye transmits and focuses light and is located behind the iris attached to the wall of the eye by suspensory ligaments known as the zonules. The lens consists of a more rigid central nucleus surrounded by peripheral cortical material, which has a softer consistency. A fine membrane known as the capsule contains the entire lens.
Cataract formation refers to a loss of transparency of the crystalline lens of the eye and is a common occurrence with age. This results in a progressive reduction in vision, which can be restored with surgery. Cataract surgery involves removal of the cataractous lens and insertion of a plastic intraocular lens to replace the crystalline lens. Removal of the cataractous lens is accomplished using ultrasonic energy to fragment and aspirate the lens by a technique known as phacoemulsification.
During such surgery, a central opening is formed in the anterior portion of the capsule to permit access to the lenticular material. An ultrasonic handpiece, typically including a needle having an outer wall and central lumen, is then inserted, contacted against and caused to fragment the lens. An elastomeric sleeve surrounding the needle provides a conduit irrigating the eye to replace material aspirated through the needle. Once the nuclear material of the lens has been removed with the assistance of ultrasonic energy, softer cortical material may be aspirated with an irrigation/aspiration cannula.
In both phases of the procedure it is important that the anterior chamber is maintained at a positive pressure and constant volume to prevent collapse, so as to prevent trauma to sensitive ocular tissues. Contact with the endothelial cells lining the posterior surface of the cornea or the iris can result in irreparable damage. Even more common is inadvertent contact or aspiration of the posterior capsule, which prevents the escape of the fluid contained in the posterior chamber of the eye known as the vitreous humour. Such inadvertent contact may result in rupture of the posterior capsule membrane.
Rupture of the posterior capsule and loss of the vitreous humour increases the risk of retinal detachment and cystoid macular oedema after cataract surgery, with subsequent loss of vision. Furthermore if the posterior capsule is disrupted during surgery it may not be feasible to properly place an intraocular lens in in the capsular bag remnant of the original lens, again resulting in a less favorable outcome than might be anticipated in uncomplicated surgery.
Maintaining a stable pressure and volume in the anterior chamber when performing phacoemulsification is of paramount concern. Optimal fluid dynamics implies sustaining a stable pressure and volume in the anterior chamber when performing phacoemulsification. Aspiration of fluid from the anterior chamber must be balanced by adequate infusion. The desired state of fluid balance may be summarized in the equation: Fi=Fo−Inflow (Fi) should equal Outflow (Fo). To avoid chamber collapse the pressure in anterior chamber (Pac) also must be greater than atmospheric pressure (Pa) and greater than vitreous pressure (Pv)−Pac>Pa>Pv.
The pressure in the anterior chamber depends on the infusion pressure, which is the difference between the irrigation pressure head (Pi), related to the irrigation bottle height, and the drop in pressure due to resistance to the inflow of irrigation fluid (Pd)−Pa=Pi−Pd. The anterior chamber pressure preferably should be maintained at a constant level to avoid alterations in chamber volume, which manifest as an unstable chamber during surgery.
A conventional apparatus used in cataract surgery includes a console containing a pump system used to generate vacuum and flow as well as the electrical circuitry that provides energy and control for the phacoemulsification handpiece. The pump systems are connected to the phacoemulsification handpiece and irrigation and aspiration cannula by tubing so that fluid and lens material can be aspirated from the eye.
Several types of pump systems are known for providing aspiration of fluid and lens material during phacoemulsification and cortical aspiration. The first type are positive fluid displacement pumps, such as a peristaltic pump. In such systems, fluid flow is generated by drawing suction through the tubing and significant vacuum may be achieved if the tubing becomes occluded. In other pump systems, such as a venturi pump, suction is generated in a cassette and the subsequent flow and aspiration of fluid from the eye is related to that preset suction level.
For either pump system, the sequence of removal of nuclear and cortical material is similar. Fluid is aspirated from the anterior chamber via suction applied through the phacoemulsification needle or irrigation/aspiration cannula and the associated aspiration tubing. This suction attracts nuclear or cortical material to the needle or cannula and may result in larger fragments occluding the tip or aspiration port.
The suction level within the tubing then rises until the negative pressure generated overcomes the resistance of the lenticular material, which is then aspirated down the tubing. This in turn causes a rapid equalization of pressure between the anterior chamber and the rest of the system, with a concomitant rapid increase in flow and drop in chamber pressure. This phenomenon is typically referred to as “post occlusion surge” and may cause a forward movement of the posterior capsule as the chamber pressure and volume fluctuates.
Vacuum applied by the phacoemulsification handpiece may be modulated by foot pedal control, thereby causing the pump system to respond by venting or equalizing the pressure in the system either to fluid or to air. The venting, however, occurs, some distance from the handpiece and anterior chamber and there is typically a lag before the vacuum in the tubing is restored to a positive pressure and the pressure in the anterior chamber is restored to the normal resting or unoccluded level.
Accordingly, it would be desirable to reduce the surge in flow rate that occurs with rapid fluctuations in vacuum pressure associated with occlusion of the phacoemulsification needle. Such control advantageously could reduce fluctuations in chamber pressure and shorten the time to attain equilibrium pressure, thereby enhancing safety of the surgical procedure and reducing the risk of inadvertent rupture of the posterior capsule.
One potential method for reducing fluid surges in the aspiration tubing is to reduce the maximum vacuum levels that are generated by the pump system. High vacuum levels, however, are advantageous in capturing fragments of nuclear material so that the fragments may be fractured into smaller pieces. It is therefore desirable to maintain high vacuum levels while reducing the high flow rates associated with surges that occur at those high levels of vacuum.
Another approach is to increase the resistance in the aspiration tubing by reducing the lumen or increasing the length of the aspiration tubing. While reducing the lumen size may be effective, the internal diameter of typical phacoemulsification tubing is generally about 1.5 mm, and any further reduction in diameter is likely to result in obstruction of the aspiration tubing by lens material. Increasing the tubing length may be accomplished by coiling the tubing to add further hydrodynamic resistance. In both cases, however, the increased resistance of the tubing exists for all vacuum levels. This is undesirable, as it would be preferable to maintain undiminished aspirational flow rates at low vacuum levels to facilitate attraction of lens fragments prior to occlusion.
In view of the foregoing, it would be desirable to provide a phacoemulsification system including flow adaptive aspiration tubing that automatically increases flow resistance in response to higher flow rates.
It further would be desirable to provide a phacoemulsification system including flow adaptive aspiration tubing that induces turbulent flow at lower flow velocities.
It would be yet further desirable to provide a phacoemulsification system including flow adaptive aspiration tubing that provides improved anterior chamber stability at higher vacuum levels.